Methods and apparatuses for direct digital drive of a laser in a passive optical network

ABSTRACT

The present invention provides a direct digital drive for a laser in a passive optical network. The direct digital drive supplies drive current to the laser.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Patent ApplicationSerial No. 60/382,506, filed May 21, 2002, titled Methods andApparatuses for Optical Network Termination and Media Access Control ina Passive Optical Network.

FIELD OF THE INVENTION

[0002] The present invention relates to an optical network and, moreparticularly, a hardware implementation of the APON protocols, a directdigital drive of a laser diode, and measurement and control system forthe direct digital drive of the laser diode.

BACKGROUND OF THE INVENTION

[0003] A passive optical network (PON) is an optical network thatdistributes signals to multiple terminal devices using passive splitterswithout active electronics, such as, for example, repeaters.Conventionally, signal delivery over passive networks uses a variety oftransfer protocols, such as, for example, a synchronous optical network(SONET) or an asynchronous transfer mode (ATM) protocols. From time totime, the International Telecommunication Union (ITU) issuesrecommendations and standards for PONs under standard G.983.1, whichstandard is incorporated herein by reference. Generally, the presentinvention is described with relation to APON, asynchronous transfer modepassive optical networks and the associated protocols, but one ofordinary skill in the art would understand that the use of APON isillustrative of the present invention and the present invention could beused for other types of passive optical networks, for example, EPON.

[0004]FIG. 1 illustrates a conventional APON 100. APON 100 could beeither a fiber to the building (FTTB) or a fiber to the home (FTTH)network configuration. Generally, the PON system comprises an opticalline terminal (OLT) 102, at least one optical network unit (ONU) 104,and at least one network termination (NT) 106 where an end user canaccess the system using, for example, a conventional computer,processor, or the like (not specifically shown). Connections 108 o fromthe OLT 102 to the ONU 104 are fiber or optical connections andconnections 108 e from the ONU 104 to the NTs 106 are electricalconnections. Depending on the number of ONUs 104 and NTs 106, one ormore optical distribution nodes (ODN, a.k.a optical splitters andcombiners) 110 may be situated between OLT 102 and ONU 104. Generally,ONUs 104 and NTs 106 reside at the end user or subscriber location (notspecifically shown).

[0005]FIG. 9 illustrates a laser diode 902 using a conventional powersource control system 904. As shown, laser diode 902 emits useful light906 and rear facet light 908. Useful light 906 refers to lighttransmitted to connection 108 o. A light monitor 910, which could be aphotodiode, a light meter, or the like, senses the intensity of rearfacet light 908. The intensity of rear facet light 908 corresponds tothe intensity of useful light 906. Substantially simultaneously withsensing the intensity of rear facet light, light monitor 910 supplies alight level feedback signal to a laser power controller 912. Laser powercontroller 912 supplies a zero level current data signal 914 to a firstprogrammable current source 916. First programmable current source 916supplies the current necessary to drive laser diode 902 at the lightintensity that corresponds to a logic level zero. Laser power controller912 also supplies a modulation current data signal 918 to a secondprogrammable current source 920. The modulation current data signal 918determines the light intensity of the useful light output 906. Amodulation signal 922 is supplied to the gate of a bi-stable switch 924to turn the switch on and off based on whether the useful lightintensity 906 should be at the logic 1 or the logic 0 intensity. Thebi-stable switch passes current from the programmable current source 920to be summed with the current from programmable source 916. The sum ofthe two currents drives laser diode 902. The feedback signal to laserpower controller 912 allows fine-tuning of the drive currents so theaverage intensity of the light signal remains within the protocolrequirements for logic levels 1 and 0. These current vary widely fromlaser diode to laser diode ranging from as low as 2 or 3 milliamps to ashigh as 50 to 60 milliamps.

[0006]FIG. 10 is a diagrammatic representation of useful light intensityto drive current. In particular, FIG. 10 shows transmission of aninformation cell 1002. As is known in the art, cell 1002 is an ATMprotocol for transmitting information. FIG. 10 (and FIG. 11) does notactually represent transmission of a complete cell of information, butrather a short burst of information for convenience. Cell 1002 arepresents drive current for exemplary cell 1002 and cell 1002 brepresents light intensity for exemplary cell 1002. As shown, cell 1002a can be considered in discrete parts 1004 a and 1004 b. Part 1004 a isthe drive current necessary for the transmission of light bearinginformation having an intensity of logic 1s and 0s. Part 1004 b is thedrive current for the transmission of light having an intensity of logic0 to allow for a zero level measurement only; in other words, noinformation is being transmitted during the zero level measurement. Theduration and timing of part 1004 b is generally controlled by theassociated transmission protocols. Similarly, light intensity shown bycell 1002 b over the course of cell 1002 transitions between the highand low drive currents for the laser diode. As the diagram shows,because of difficulties in controlling the drive current for the laserdiode, first logic pulse 1006 is typically wasted adjusting the drivecurrent for the existing operating conditions and temperatures. Part ofthe difficulty of controlling current occurs because the lasing cavityneeds to charge the photons sufficiently to begin emitting light. Also,when the photons in the lasing cavity are sufficiently charged to thethreshold or knee level, the laser emits a burst of light and oscillatesuntil the photons are properly charged and the laser is correctlyoperating above the threshold level.

[0007] As can be seen from FIG. 10, during non-transmission period 1008,laser diode 902 is driven at a 0 current. Laser diode 902 is driven witha 0 current to inhibit the accidental transmission of light from laserdiode 902 when laser diode 902 does not have a transmission grant. Thedrive current for a logic level 0 light intensity is some currentgreater than 0 current. Thus, one reason the first logic pulse 1006 iswasted is that time during the transmission of cell 1002 is required tocharge the photons in the laser. While maintaining the laser drivecurrent at the zero logic drive current (which is greater than 0 amps)would maintain the laser charged, it might allow for inadvertent lightemission from the laser, which would cause interference with othertransmitting lasers.

[0008] Transmission of a cell of information will be further explainedwith reference to FIGS. 1-3. Using ATM protocols, OLT 102 receivesincoming cells 202 of information from a transport network or servicenode 112 destined for NTs 106. For simplicity, this example has threecells of information ABC destined for three separate NTs 106. Thetransport network could be any style network, such as the Internet, aPlain Old Telephone Service (POTS), digital video and/or audio streams.OLT 102 routes the incoming cell 202 over optical connection 108 othrough ODN 110 to three ONUs 104 ₁₋₃. Using conventional protocolsassociated with APON, ONU 104 ₁₋₃ selects the data for its associated NT106 and converts the optical signal to an electrical signal fordistribution to the NT 106 over connection 108 e. For example, ONU_(a)selects data cell A from incoming cell 202 and converts that data intoan electrical signal for NT 106.

[0009]FIG. 3 shows the transmission of outgoing information from two NTs106 ₄ and 106 ₅, for example. NT 106 ₄ transmits an outgoing data cell Dand NT 106 ₅ transmits an outgoing data cell E over connection 108 e toONUs 104 ₄ and 104 ₅. The ONUs 104 ₄ and 104 ₅ converts the electricalsignal to an optical signal for transmission to ODN 110 over connection108 o. ODN 110 combines the data cells D and E into a single cellstream. 302. To prevent data collisions, APON protocols require ONUs 104to transmit data cells at specific times and in short bursts. Thus, thelaser diode (not specifically shown) associated with ONU 104 typicallytransmits a cell lasting a fraction of a microsecond or a burst of cellslasting a few microseconds, but may only transmit infrequently. Also,the laser diode typically transmits regularly, but may be powered downfor an indeterminate length of time, which may change the laser diode'soperating characteristics including the laser diode's operatingtemperature. Because the subscriber side laser diode may only transmitdata infrequently and when it does transmit data the transmission isonly a short burst of information, it is difficult to control the laserpower input and light intensity.

[0010] As can be seen from the above, it would be beneficial to provideimproved methods and apparatuses for measuring and controlling the powerand intensity of the laser diode associated with the passive opticalnetworks.

SUMMARY OF THE INVENTION

[0011] The foregoing and other features, utilities and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention as illustrated inthe accompanying drawings.

[0012] To attain the advantages and in accordance with the purpose ofthe invention, as embodied and broadly described herein, apparatuses todrive a laser in a passive optical network are provided. Apparatusesgenerally include a laser with a power supply controller coupled to aprogrammable logic device (commonly a field programmable gate array).The power supply controller transmits a drive signal to the programmablelogic to turn on output pins. The drive current supplied to the laserdepends on the combination of output pins turned on.

[0013] The present invention further provides methods for driving alaser in a passive optical network. The methods include generating alaser drive signal from a power supply controller and receiving thelaser drive signal at a programmable logic device. At least one outputpin of the programmable logic device is turned on based on the signal tosupply current to the laser.

BRIEF DESCRIPTION OF THE DRAWING

[0014] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate some preferredembodiments of the invention and, together with the description, explainthe goals, advantages and principles of the invention. In the drawings,

[0015]FIG. 1 is functional block diagram of a conventional passiveoptical network;

[0016]FIG. 2 is a functional block diagram of a conventional passiveoptical network transmitting information from the transport network tothe network terminations;

[0017]FIG. 3 is a functional block diagram of a conventional passiveoptical network transmitting information from the network terminationsto the transport network;

[0018]FIG. 4 is a functional block diagram of a laser diode powercontrol feedback loop in accordance with one aspect of the presentinvention;

[0019]FIG. 5 is a flowchart 500 illustrative of powering a laser inaccordance with one aspect of the present invention;

[0020]FIG. 6 is a functional block diagram of a direct digital drive fora laser in accordance with one aspect of the present invention;

[0021]FIG. 7 is a flowchart 700 illustrative of controlling a laserdrive in accordance with one aspect of the present invention;

[0022]FIG. 8 is a functional block diagram of an ONU in accordance withone aspect of the present invention;

[0023]FIG. 9 is a functional block diagram of a prior art laser diodepower supply;

[0024]FIG. 10 is a graphical representation of prior art laser currentto light characteristics; and

[0025]FIG. 11 is a graphical representation of laser current to lightcharacteristics consistent illustrative of aspects of the presentinvention.

DETAILED DESCRIPTION

[0026] With reference to FIGS. 1-11, the present invention will bedescribed. It is intended that all matter contained in the descriptionbelow or shown in the accompanying drawings shall be interpreted asillustrative and not in a limiting sense.

[0027]FIG. 4 shows a laser diode 400 illustrative of the presentinvention. While laser diodes are the typical lasing device for passiveoptical networks, other lasing devices could be used. Laser diode 400includes a first lasing side 402 and a second side 404. First lasingside 402 emits light to the passive optical network (not shown in FIG.4) while second lasing side 404 emits light to an intensity monitor 406,such as a photodiode or the like. Intensity monitor 406 sends a feedbacksignal to control a power source 408 that drives the laser diode 400.Thus, for example, when transmitting a logic 1 from laser diode 400, thelaser diode 400's light intensity must be a certain intensity. Intensitymonitor 406 would measure the intensity of a light beam 410. If theintensity of light beam 410 is below the logic 1 threshold, then thepower source increases the power input to laser diode 400. Increasingthe power input to laser diode 400 increases the intensity of light beam410. Similarly, if the intensity is too high, the power is decreased. Alogic 0 transmission would be controlled in a similar fashion.

[0028] The power source typically uses analog components, such as abi-stable analog switch, to provide power to the laser. The analogcomponents control power to the laser diode, which in turn controls thelight intensity out of the laser. The analog power source, as referencedabove, is a conventional device and will not be further explained.

[0029] The power necessary to drive the laser so that the lightintensity is the correct level, however, is a function of widedevice-to-device characteristics, the laser's aging characteristics, andthe laser's operating temperature. Further, if the laser isinadvertently overpowered, the laser can be damaged. Thus, most currentsystems waste the initial portion of the lasing period in order to rampthe light intensity up to the desired level. Further, the analog powersource design is a complex solution that requires numerous, relativelyexpensive parts. Finally, the analog power source provides rapidtransition between the logic 1 and the logic 0 states, but does notconveniently provide a means to rapidly vary each of the currents to thelaser at the logic 1 or 0 currents, respectively.

[0030]FIG. 5 shows a flowchart 500 indicative of a method of initiallypowering the laser diode 400 so that on its next transmission, theinitial current supplied by power source 408 is approximately thecorrect power for the desired light intensity. First, during a previoustransmission of signal from laser diode 400, the previous transmissionof power requirements for a logic 0 signal and a logic 1 signal arerecorded, step 502. While the previous transmission power requirementscan be stored as analog data, if the analog data is converted to digitaldata, the information can be maintained for a longer duration, which maybecome important depending on the time delay between the previous andsubsequent data transmissions. The previous transmission powerrequirements are then normalized to a predetermined temperature, such as25° C., step 504. The normalized power requirement to generate a lightintensity for logic 0 and the power requirement to generate a lightintensity for logic 1 are stored, step 506. Notice, if the previoustransmission power requirements are not normalized, the pervioustransmission actual temperature could be stored, as will be explainedfurther below. Next, the laser diode is inactive for an indefiniteperiod of time, which could be several microseconds, seconds, minutes,hours, days, weeks, months, years, etc., step 508. The inactive periodis generally contemplated to be a time when the laser diode is turnedoff, but the laser diode can be inactive for any number or reasons.Eventually, the laser diode is required to transmit a logic 0, a logic1, or some combination of logic 0s and 1s in a subsequent transmission,step 510. At the time the subsequent transmission is required, atemperature sensor records subsequent transmission temperature, step512. Using the subsequent transmission temperature, subsequenttransmission power requirements for transmission of a logic 0 and alogic 1 are calculated from the stored normalized power requirements,step 514. As mentioned above, if the previous transmission powerrequirements are not normalized, the previous transmission temperature,the subsequent transmission temperature, and the previous transmissionpower requirements are used to calculate the subsequent transmissionpower requirements. The laser diode 400 is driven by the calculatedsubsequent transmission power requirements, step 516. Typically, thelaser diode 400 is not precisely driven, so conventional feedback loopsare used to fine tune the power needed to drive the laser diode to thecorrect intensities, step 518. Because the calculated subsequenttransmission power requirements for the laser diode are sufficientlyclose to the actual power requirements, less time for each transmissionburst is wasted adjusting the laser power to provide the correctintensity. Further, by applying approximately the correct drive power,the laser diode is less likely to be damaged by overdriving. Finally,during the subsequent transmission, new previous power requirements arerecorded normalized, and stored for logic 0 and logic 1 transmissions,step 520, and the laser enters an inactive period, step 508. CurrentAPON protocols identify at what points during transmission of a cell thepower levels for logic 0 and/or logic 1 should be measured. Notice, newprevious power requirements do not need to be obtained each subsequenttransmission. Rather, it is possible to store power requirements on sometime interval or some predetermined number of transmission, althoughreplacing the stored power requirements each transmission would likelyprovide better initial power estimates for each subsequent transmission.

[0031] Direct Digital Drive

[0032] As mentioned above, conventional power sources to drive laserdiode 400 utilize analog components. The analog solution components arerelatively expensive and relatively numerous. In other words, as shownin FIG. 9, the digital signal from the processor 912 is supplied toanalog power sources 916, 920. A control signal to the bi-stable switch924 causes analog current to be supplied to the laser. It would bebeneficial to supply the control signal from processor 912 to digital toanalog (“DAC”) converter (not specifically shown in FIG. 9). The DACwould use the digital control signal to supply an analog current to thelaser. While a conventional DAC could be used, one of skill in the artwould recognize that other converters are possible, such as ASICsilicon, customized microchips, or specialty DAC components. One ofskill in the art would further recognize that programmable logic, suchas field programmable gate arrays, are also possible. The below relatesto the use of programmable logic, but one of skill in the art wouldrecognize on reading the below, that multiple converters are possible.

[0033] Using resistors and logic for a direct digital drive requiresseveral relatively cheap resistors and the use of part of a programmablelogic part, such as an FPGA, that is conventionally available in a PONtransceiver and typically has some un-used logic. FIG. 6 is a functionaldiagram of a digital power source 600 for the laser. Current embodimentsof the digital power source 600 comprise a field programmable gate array(FPGA), but as one of ordinary skill in the art would recognize onreading this disclosure, other digital devices could be used. FPGA 600includes a number of pins 602 _(1-n) and a number of resistors 604_(1-m). Generally, each pin has a corresponding resistor. Each pin 602of FPGA 600 is driven by a particular voltage, such as, for example, 3volts. For pin 602 ₁ to supply, for example, 1 milliamp of incrementalcurrent drive to the laser (not shown in FIG. 6), then resistor 604 ₁would be 3K ohms. For pin 602 ₂ to supply, for example, 2 milliamps ofincremental current drive to the laser, then resistor 604 ₂ would be1.5K ohms. For pin 602 ₃ to supply, for example, 4 milliamps ofincremental current drive to the laser, then resistor 604 ₃ would be 750ohms. For pin 602 ₄ to supply, for example, 8 milliamps of incrementalcurrent drive to the laser, then resistor 604 ₄ would be 375 ohms, etc.Thus, if the laser required 13 milliamps to supply a logic 1 lightintensity, pins 602 ₁, 602 ₃, and 602 ₄ would be used to drive thelaser. For 10 milliamps, pins 602 ₂ and 602 ₄ would be used to drive thelaser. As one of ordinary skill in the art would recognize on readingthis disclosure, any number of combinations of output voltage andresistors could be used to provide the required combination of potentialvoltages. Further, while the above is shown using 1 milliamp stepssmaller or larger increments could be designed for as a matter of designchoice. Of course, instead of using a binary progression as shown, eachpin could be capable of the same currently. For example, each pin 602could each drive 1 milliamp. Still further, a combination of a binaryprogression and equally weight currents could be used. Still further,binary groupings would be possible, such as groups of 4 pins supply 1 to15 milliamps. In other words, each group of pins has a 1 milliamp pin, a2 milliamp pin, a 4 milliamp pin and an 8 milliamp pin. In this case,two groups would be used to supply 16 milliamps, which could all pins of1 group and 1 milliamp from another, or 2 8 milliamp pins, etc. Toprovide, for example, up to 50 milliamps, four groups of pins would beprovided. Of course, other combinations could be used as a matter ofdesign choice and availability. For even greater control, adding pulsewidth modulation and filtering capacitance on one or more pins tomaintain a relatively consistent DC output for one or more pins providesfurther precision control of the current drive. The additional precisionis achieved, in part, because the pin's DC output could be controlledbetween essentially zero milliamps and the maximum milliamps.

[0034] Referring back to FIG. 9 and FIG. 6, laser diode 902 and lightmonitor 910 supply a feedback signal to laser power controller 912.Laser power controller 912 supplies a control signal that turns onspecific pins of direct digital driver 600 to supply current to thelaser diode. Notice, laser power controller 912 could be incorporatedinto the digital driver 600 by, for example, using programmable logic ofa FPGA.

[0035] Controlling Intensity and Power

[0036] Using the FPGA and digitally driving the laser, it is possible tovary the current driving the laser instantaneously on a bit by bitbasis. Thereby imparting a novel flexibility in driving the laser. Forexample, APON protocols dictate that the logic 0 light intensity be 0.Controlling the laser at 0 intensity also prevents light from anon-transmitting ONU to collide with an ONU having a grant to transmit.However, laser diodes operate in a nonlinear manner at low intensitylevels. For example, if the laser light intensity is 0 at 5 milliamps,then the laser light intensity is 0 at 2 milliamps, also. This makes itdifficult to measure the power levels required for logic 0 lightintensities, because of inefficiencies in the feedback.

[0037] Analog drives using bi-stable switches to drive the laser providetwo currents. The bi-stable switch provides a first current for thelogic 0 intensity, and a second current for the logic 1 intensity. Thebi-stable switches rapidly switch the drive current between the 0 leveldrive and the 1 level drive. However, the analog switch is relativelyinflexible. In other words, it is difficulty to change the suppliedcurrent at either the 0 or 1 level. Using the direct digital drive,however, it is possible to instantaneously alter the laser's drivecurrent. The variation is not only between the 0 and 1 level, butvariations between 0 and 1, such as, for example, 2% intensity, 5%intensity, 75% intensity, or any current between 0 and maximum drive.For example, when attempting to measure the non-linear 0 intensitypoint, it is possible to drive the laser at a level slightly above 0intensity level, such as, for example using 15% light intensity. Usingthe drive current level corresponding to light intensity just above 0,it is possible to extrapolate the correct drive current for zerointensity, or a particular below zero intensity current.

[0038]FIG. 7 shows a flowchart 700 exemplary of this procedure. First,during the period when the logic 0 light intensity power requirementsare measured, the laser would be supplied sufficient power to drive itat 15% intensity, step 702. The power requirement would be measured,step 704. Using the power required to drive the laser at 15%, the powerrequirement for 0% intensity would be calculated, step 706. Thecalculated current would be the current used to subsequently drive thelaser to a zero level.

[0039] Another benefit of the unique controller it can “under drive” thelaser. To under drive the laser means supplying power to the laserslightly below its 0 threshold level. For example, if the 5 milliampcurrent drives the laser at a 0% intensity light, under driving thelaser may be done by supplying 4 milliamps or at least some current lessthan 5 milliamps. By under driving the laser, the laser could be turnedon at a level below its 0 level by a predetermined amount. Turning thelaser on below the 0 level essentially eliminates any risk ofaccidentally emitting light from the laser. Thus, the laser could beunder driven prior to its designated transmission time. By under drivingthe laser, the laser has at least partially charged the photons in thelasing cavity. Thus, less time will be required to actually increase thephoton density the remaining amount to the lasing threshold density andactually turn on the laser.

[0040]FIG. 11 is a diagrammatic representation of useful light intensityto drive current showing the above described control scheme. Similar toFIG. 10, FIG. 11 represents the transmission of an exemplary cell ofinformation 1102. Cell 1102 a represents drive current for exemplarycell 1102 and cell 1102 b represents light intensity for exemplary cell1102. As shown, cell 1102 a can be considered in discrete parts 1104 aand 1104 b. Part 1104 a is the drive current necessary for thetransmission of light having an intensity of logic 1s and 0s for thetransmission of cell information. Part 1104 b is the drive current forthe transmission of light having an intensity of logic 0 to allow for azero level measurement. (A similar period exists for monitoring thelogic 1 level, not shown). The duration and timing of part 1104 b isgenerally controlled by the associated transmission protocols.Similarly, light intensity shown by cell 1102 b over the course of cell1102 transitions between the high and low drive currents for the laserdiode.

[0041] As shown in FIG. 11, non-transmission period 1108 has two partsalso, part 1108 a is a period when no current is supplied to the laser.Part 1108 b is a pre-drive period. In the pre-drive period, the digitaldrive is calibrated to supply a current sufficiently below the 0 logiclevel current. This allows the photons in the lasing cavity to bepartially charged prior to when the laser receives the grant to transmitthe cell 1102. Because the laser can be pre-charged for a predeterminedtime prior to receiving the transmission grant, the first light pulse ofdata 1106 can be used. In contrast, prior art power sources required atleast the first pulse 1006 to be wasted because the laser had to ramp upto operating conditions.

[0042] Further, as mentioned above, measuring the current necessary todrive the laser at a 0% light intensity is difficult due to thenon-linear nature of lasers below the 0% threshold. Thus, using thedigital drive it is possible during part 1104 b of the cell transmissionto measure the current needed to drive the laser at, for example, 15%intensity. This light intensity during part 1104 b of the celltransmission is shown by part 1110. Notice while 15% is stated, otherpercentages could be used, such as 2% light intensity, 5% lightintensity 50% light intensity, 78% light intensity, etc. It is possibleto measure the drive current 1112 used to produce light intensity 1110and then calculate the drive current that would be necessary to drivethe laser at only just a 0% light intensity. The calculated drivecurrent would be used to send logic 0 signals and to pre-drive the laserat some predetermined current below the logic 0 current.

[0043] Implementation of Functionality in FPGA

[0044] Referring to FIG. 8, section 8 of the G.983.1 standard indicatesseveral functionalities for ONU transceivers 800. FIG. 8 shows aconventional ONU transceiver 800. Transceiver 800 includes atransmission portion 802 and a receiving portion 804 to send and receiveoptical signals. Transceiver 800 typically contains a hardware portion806, such as conventional synchronization circuitry 808, switchingcircuitry 810, signaling circuitry 812, and a user interface circuitry814 and a software portion 816. Traditionally, a microprocessor is usedto implement software-based controls of the G.983.1 standard, such asdata verification and security, such as churning. It has beendiscovered, however, that significant advantages are obtained when themicroprocessor based software controls are replaced with fieldprogrammable logic 820, a.k.a. a FPGA. Any conventional method can beused to program the programmable logic of the FPGA to perform thefunctions conventionally performed by the microprocessor-based software.As shown, the FPGA 820 can be a single chip that encompasses the boththe conventional hardware (circuits 806, 808, 810 and 812) as well asthe conventional software portion 816 normally executed by amicroprocessor.

[0045] Using the FPGA to provide the software functionality providesseveral advantages. First, using the FPGA logic allows a single chip(the FPGA chip) to control the PON operation. Using one chip to controlthe PON reduces costs, power consumption, space, etc. Further, using onechip provides increased reliability.

[0046] Using the FPGA also avoids typical parallel efforts betweenhardware and software development engineering groups (with their wellknown communication delays, and difficulties with integrating andtesting two disparate design parts). Moreover, updates to the hardwareand software can be provided in one combined image. Image is a term foran array of data bytes used to program a device, such as a FPGA. Oftensoftware is updated in the field by downloading an image, in this casefrom OLT 102 to ONU 104. ONU 104 would then use the new image to updateitself, without, of course, replacing its memory. Occasionallyprogrammable hardware is updated with a new image in the above fashion.The new hardware often needs software to recognize the new hardware. Byusing a single FPGA to do both the hardware and software functions, thesoftware and hardware updates can be transmitted using a single image.

[0047] Using programmable hardware also provides unique operationaladvantages. In particular, performing multiple tasks in a microprocessorbased software is, in reality, performing a little piece of each task inserial, choosing suitably small pieces to give the illusion that theyare happening simultaneously. Implementing the software in the FPGA,however, allows parallel operation using separate logic for eachfunction. Using separate logic paths can be faster. Further, parallelprocessing in the FPGA has a higher reliability than parallel processingin a software environment. Generally, the FPGA has a lower probabilityof getting hung up. In other words, because the functions are performedon separate logic paths, there is less chance of one process holding upanother, although both still only do one little piece of one thing at atime and if the sequence gets lost the whole machine needs restarting.With separate entities if one gets lost the other processes carry onunaffected, and there is a good chance the lost process will get sortedout again.

[0048] Finally, a hardware implementation in CMOS generally uses lesspower then an equivalent function implemented in a microprocessor basedsoftware. The FPGA requires less power because the same functionalitycan be programmed using fewer gates. Also, gates can be toggled asrequired, thus less gate charge is moving. Less gate charge means lesscurrent and energy. For example, a 16 bit counter in hardware where 16logic flip-flops hold 16 bits of data in place. They each are toggledonly as required to make the count. A general-purpose processor,however, may store 16 bits of data in a 32-bit memory until required.The 32 bits (many of which are unused in this example, then are routedout of memory, through the general purpose logic and back to memory toachieve the same function.

[0049] While the invention has been particularly shown and describedwith reference to a preferred embodiment thereof, it will be understoodby those skilled in the art that various other changes in the form anddetails may be made without departing from the spirit and scope of theinvention.

I claim:
 1. A direct digital drive for a laser in a passive opticalnetwork, comprising: at least one laser; at least one power supplycontroller; at least one programmable logic device; the at least oneprogrammable logic device comprises at least one output pin; the atleast one power supply controller transmits at least one laser drivesignal to the at least one programmable logic device; the at least oneprogrammable logic device turns on the at least one output pin based onthe transmitted at least one laser drive signal; the at least oneprogrammable logic device supplies current to the at least one laserfrom the at least one turned on output pin so that the at least onelaser outputs at least one light signal at at least one light intensity.2. The direct digital drive according to claim 1, comprising: at leastone light measuring device capable of measuring the at least one lightintensity output from the at least one laser; and at least one feedbacksignal generator that generates at least one feedback signal based onthe measured light intensity, wherein the at least one power supplycontroller adjusts the at least one laser drive signal based on the atleast one feedback signal.
 3. The direct digital drive according toclaim 2, wherein the at least one light measuring device is aphotodiode.
 4. The direct digital drive according to claim 1, whereinthe at least one programmable logic device is a field programmable gatearray.
 5. The direct digital drive according to claim 4, wherein the atleast one output pin comprises a plurality of output pins; and theplurality of output pins supply a predetermined voltage when turned on;and at least one resistor is coupled in series between each of theplurality of output pins and the at least one laser, such that each ofthe plurality of output pins supply a predetermined current to the atleast one laser.
 6. The direct digital drive according to claim 5,wherein the plurality of output pins supply the same predeterminedcurrent.
 7. The direct digital drive according to claim 5, wherein atleast one of the plurality of output pins supplies a differentpredetermined current.
 8. The direct digital drive according to claim 7,wherein the plurality of output pins supply output current in a binaryprogression.
 9. The direct digital drive according to claim 5, whereinat least one of the plurality of output pins is coupled to an RC circuitto supply a tunable current.
 10. A method for transmitting a cell ofinformation in an optical network by digitally driving a laser, themethod comprising the steps of: generating a laser drive signal from apower supply controller; receiving the generated laser drive signal at aprogrammable logic device; turning on at least one output pin of theprogrammable logic device based on the received laser drive signal, suchthat the at least one output pin turned on provides current to a laserso that the laser outputs a light signal.
 11. The method according toclaim 10, further comprising the step of: measuring an intensity of thelight signal output from the laser; supplying a feedback signal to thepower supply controller; and adjusting the generated laser drive signalbased on the feedback signal.
 12. The method according to claim 10wherein the generating a laser drive signal comprises the steps of:generating a first laser drive signal corresponding to a first lightintensity; and generating a second laser drive signal corresponding to asecond light intensity; the first light intensity corresponding to alogic 1 signal; and the second light intensity corresponding to a logic0 signal.
 13. The method according to claim 12, wherein the turning onat least one output pin of programmable logic comprises the steps of:turning on at least a first output pin of the programmable logic devicecorresponding to the first laser drive signal; and turning on at least asecond output pin of the programmable logic device corresponding to thesecond laser drive signal, such that at least the first output pin is adifferent combination then at least the second output pin.
 14. Themethod according to claim 10 wherein the output pins of the programmablelogic device provide different currents to drive the laser.
 15. Themethod according to claim 12, further comprising the step of: storing afirst current value necessary to drive the laser to produce the firstlight intensity; storing a second current value necessary to drive thelaser to produce the second light intensity; wherein the step ofgenerating the first laser drive signal uses the stored first currentvalue and the step of generating the second laser drive signal uses thestored second current value; and
 16. The method according to claim 15,further comprising the steps of: measuring an intensity of the lightsignal output from the laser; supplying a feedback signal to the powersupply controller; and adjusting the generated first laser drive signaland the generated second laser drive signal based on the feedbacksignal.
 17. The method according to claim 16, further comprising thestep of: replacing the stored first current value and the stored secondcurrent value with a new first current value and a new second currentvalue, wherein the new first current value and the new second currentvalue are determined from the supplied feedback signal.
 18. The methodaccording to claim 15, further comprising the step of: normalizing thestored first current value and the stored second current value.
 19. Adirect digital drive for a laser in a passive optical network,comprising: means for generating a light signal; means for driving themeans for generating the light signal; means for controlling the meansfor driving, the means for controlling generates a drive signal; and themeans for driving uses the generated drive signal to determine a currentto be supplied to the means for generating the light signal so that thelight signal corresponds to at least one intensity.
 20. The directdigital drive according to claim 19, wherein the means for generating alight signal comprises a laser diode.
 21. The direct digital driveaccording to claim 19, wherein the means for controlling is a processor.22. The direct digital drive according to claim 19, wherein the meansfor driving comprises at least one of a programmable logic, an ASICsilicon, a microchip, and a digital to analog converter.
 23. A directdigital drive for a laser in a passive optical network, comprising: atleast one laser; at least one power supply controller; at least onedigital to analog converter; the at least one power supply controllertransmits at least one laser drive signal to the at least one digital toanalog converter; the at least digital to analog converter generates ananalog drive current based on the at least one laser drive signal; theat least one digital to analog converter supplies current to the atleast one laser so that the at least one laser outputs at least onelight signal at at least one light intensity.
 24. The direct digitaldrive according to claim 23, wherein the at least one digital to analogconverter comprises at least one of a DAC, ASIC silicon, microchips, andprogrammable logic.
 25. The direct digital drive according to claim 23,comprising: at least one light measuring device capable of measuring theat least one light intensity output from the at least one laser; and atleast one feedback signal generator that generates at least one feedbacksignal based on the measured light intensity, wherein the at least onepower supply controller adjusts the at least one laser drive signalbased on the at least one feedback signal.
 26. The direct digital driveaccording to claim 25, wherein the at least one light measuring deviceis a photodiode.
 27. The direct digital drive according to claim 23,wherein the at least one digital to analog converter is a fieldprogrammable gate array.